Automatic gain control



April 21, 1942. T. a. D. TERRONI 7 AUTOMATIC GAIN CONTROL Fil ed Nov.20, 1940 Patented Apr. 21, 1942 AUTOMATIC GAIN CON TBOL Teseo BrunoDante Terroni, Broadgreen, Liverpool, England, assignor to AutomaticTelephone & Electric Company Limited, London, England, a British companyApplication November 20, 1940, Serial No. 366,468 In Great BritainDecember 11, 1939 6 Claims.

The present invention is concerned with improvements in or relatingtocarrier telephone and like carrier signalling systems, and is moreparticularly concerned with automatic gain con- 1 trol arrangements foruse therein.

In systems of the above type it is the usu practice to employ atintermediate points and possibly also at terminal points, thermionicrepeaters by means of which the received speech and/or signal energy isamplified before it ispassed on along the line.

Under substantially constant conditions and for any given length ofline, the gain of the various amplifiers can be set so that in spite ofthe line attenuation the received energy will be of the same value asthe input energy at-the transmitting end.

Due to variations in weather conditions and temperature, etc., theline'attenuation does not remain constant and it is therefore necessaryto introduce some form of gain control at the amplifier points. This canbe done manually but this method is not very satisfactory in view of thefact that constant supervision is necessary to enable the gain to bereadjusted each time line variations occur. It is the general object ofthe present inventionto provide a simple and improved method ofautomatic gain control which employs only well tried components.

According to the invention, for the purpose of adjusting the gain ofamplifier equipment in accordance with the line conditions 'a current ofconstant value and having a frequency which lies just outside,preferablyin an unused portion of the carrier frequency range transmitted, istransmitted over the same medium as the carrier speech and/or signallingcurrents, the amplitude of the current at the point of reception or gaincontrol serving to adjust the gain control in inverse ratio so as tomaintain the strength of output currentequal or in some other definiterelation to the input current with varying line conditions. Convenientlythe current is measured by means of a meter at each point at whichautomatic gain control is to be provided, the reading of the meter beingscanned by a light ray and the arrangements are such that in accordancewith the reading of the meter a number of impulses are generated whichstep a switch-to a position connecting with one of a number of tappingson a gain control rheostat in such manner that the output energytransmitted from each amplifier station will remain at the pre-set valueirrespective of variations in the input energy received thereat.

In order. to prevent hunting of the gain control equipment, the scanningof the meter reading is arranged to be made at a slow speed and twosetting switches are provided which respond alternatively to the seriesof pulses representative :of the meter reading, a switch after havingbeen set maintaining its position until the second switch has been setby thev next series of impulses and then restoring to normal inreadiness for afurther series of impulses and so on.

The invention will be better understood from the following descriptionof one method of carrying it into eflect, reference being had to the.accompanying drawing of which Fig. 1 shows schematically sumcient ofthe meter circuits and the gain control circuits to enable the inventionto be readily understood; and Fig. 2 shows a known manner of associatingthe meter with the thermionic valve by which the gain control circuitsare controlled and is substantially identical with the drawing ofBritish specification No.

Considering now the operation and referring to Fig. 1 of the drawing, acurrent having the frequency allotted to the pilot channel when receivedover the line conductors l0 and I 1 passes through a filter F which istuned thereto and causesa heavily damped meter movement RM such as thatof a direct current ammeter in the anode circuit .of' a suitabledetector valve D. which movement is shunted by the condenser 12- to takeup a reading corresponding to the amplitude of the received pilotfrequency current. In connection with this current it may be mentionedthat the frequency of pilot channel will lie in the unused portion ofthe transmitter carrier frequency range, and the amplitude of the energytransmitted at the sending end will be maintained constant. The hightension supply to the anode of the valve D is completed from lead l3 viathe inductance L and the meter RM.

As regards the meter RM, illustrated in more detail in Fig. 2, this willbe of the so-called "photo-telemetering type, the construction of whichis known to those versed in the remote supervisory control art and thegeneral form and operation of which is disclosed in our prior Britishspecifications Nos. 441,881, 441,999 and 460,187. In thesespecifications are described arrangements by which meter readings aretransmitted by optically scanning an electric meter bya rotary movementof a scanning device 5 and arranging that a light ray emanating from alamp 9 and deflected-by a prism I so as to be extended on to aphoto-electric cell Ill, and intermittently interrupted by a comb Ihaving non-reflecting teeth against a reflecting surfaced member 3 for acertain portion of the cycle, and opened for another portion of thecycle by a vane which is moved to correspond to the reading of theinstrument. In the prior specifications the proportions of the twoperiods correspond to the reading of the meter but in the presentarrangement use is made merely of the number of impulses produced ineach series in accordance with the number of comb teeth and slotsuncovered. These pulses which are generated by the photoelectric cell inresponse to the intermittent interruption of light rays extendingthereto are communicated over leads I4 and I5 to a thermionic valve Vwhich may be a vacuum tube of I the trigger gas discharge type and inthe anode circuit of which is connected a relay IMP.

Relay IMP in responding to one set of impulses steps a switch which willbe designated as switch A to a corresponding position as a result ofwhich a suitable tapping is selected on the gain control rheostat l6 viawiper and bank A4. On receipt of a second set of impulses correspondingto the next scanning operation, a switch which will be designated asswitch B is set to a position corresponding thereto and switch A, ishoned and prepared to receive the next series of impulses. The switchesA and B are of the electromagnetically operated reverse drive type, andare assumed for the purpose of the invention to have six positions,although of course in practice ten or more positions will be the moreusual, in order that a finer adjustment of the amount of gain may beeffected. Preferably the deflection of the meter RM will beapproximately proportional to the strength of the pilot frequencycurrent as measured in decibels above or below standard.

Considering now the circuit operation in more detail, it will be assumedthat both switches A and B are in their normal positions I and hencewhen relay IMP responds to the first series of impulses, positive isextended over armature impl to pulse the magnet AM of switch A, relay Poperating on the first pulse and shortcircuiting one of its windings, sothat by means of the short-circuiting effect in addition to that of itsslug it will readily hold during the impulse train.

At the end of the series of impulses switch A will have been set to aposition corresponding to the position of the meter pointer, and relay Pwill release after a short interval. At armature p2 a circuit is nowcompleted to operate relay COA over wiper and bank AI in series with themagnet AM which will neither hold nor operate under this condition.Relay COA in operating at its armature coal prepares a circuit to themagnet BM of switch B, at armature coa! completes an operating circuitover wiper A3 of switch A and normal contact and wiper B2 of switch Bfor relay COY and at armature coal allows the incoming lines it and l Ito be connected up through the amplifier H to the outgoing lines l8 andI9 via wiper and bank Al and a selected tapping on the gain controlrheostat l8. Relay COY in operating looks over its armature coyl andoperates relay RB to disconnect its initial energising circuit and toallow relay COB to be operated when the next series of impulses arereceived.

When the photo-telemeter performs a second scanning operation, a secondseries of pulses is delivered to relay IMP which in impulsing this timeis effective on the magnet BM, relay Q holdhome position I where thedriving circuit is disconnected at the wiper and bank Al.

At the same time relay COA is short-circuited by the positive extendedover armature cobl and releases so that the gain control is handed overto wiper and bank 134 over the normally closed springs associated witharmature coat. At armature cob2 relay COY is released and in turnreleases relay RB. When the A switch reaches the home position I. relayCOX is operated, locks and operates relay RA in order to remove theshort circuit from relay COA in readiness for the third series ofimpulses.

In response to the third series of impulses relay A is once again set toa position corresponding to the position of the meter pointer and theoperation will thereupon continue in the manner already described.

Circumstances leading to variation in the level of the received pilotfrequency also bring about the same variations in the received carrierfrequency level at an amplifier point in a connection, and at this pointthe apparatus according to the invention ensures that a correspondinginverse variation is made to the gain of the amplifier so that theoutput level is maintained constant irrespective of variations in theinput level thereto.

I claim:

1. In a carrier-wave signalling system, a line, a filter connected tosaid line designed to pass currents having a frequency or frequenciesclose to but outside the frequency band of the carrler-waves,variably-responsive means controlled by current passing from the linethrough said filter, an amplifier connected to said line independentlyof said filter by which the carrierwaves are amplified, gain-controllingmeans for determining the gain of said amplifier, a secondgain-controlling means for determining the gain of said amplifier andcyclically operated switching means for placing one gain-controllingmeans under the control of said variably-responsive means in inverserelationship to the strength of current flowing through said filterwhile the gain of said amplifier is controlled solely by said secondgain controlling means.

2. In a carrier-wave signalling system a line, a filter connected tosaid line designed to .pass currents having a frequency or frequenciesclose to but outside the frequency band of the carrierwaves,variably-responsive means controlled by current passing from the linethrough said filter, an amplifier connected to said line independentlyof said filter by which the carrier-waves are amplified,gain-controlling means for determining the gain of said amplifier, asecond gain-controlling means for determining the gain of saidamplifier, said gain-controlling means each including a device adaptedto be variably set by said variably-responsive means and means foralternately placing one device solely under the control of saidvariably-responsive-means while the 3. In a carrier-wave signallingsystem a line, a filter connected to said line designed to pass currentshaving a frequency or frequencies close to but outside the frequencyband of the carrierwaves, a meter controlled by current passing from theline through said filter, an amplifier connected to said lineindependently of said filter by which the carrier-waves are amplified, acomb having teeth a variable numberof which is exposed by said meteraccording to the current flowing through it, a light sensitive element,means for causing a ray of light to scan said teeth cyclically and causethe light sensitive element to generate pulses corresponding in numbertothe number of teeth exposed, a step-by-step switch including means fordetermining the gain of said amplifier arranged to be set by saidpulses, a second step-by-step switch including means for determining thegain of said amplifier also arranged to be set by pulses from saidscanning means, switching means whereby one step-bystep switch is aloneconnected up to control the gain of the amplifier while the other isbeing reset by pulses from said scanning means and controlling meansresponsive to the successive series of pulses generated by said lightsensitive element to operate said step-by-step switches alternately andperiodically to change the gain control to agree with that determinedduring the previous period.

4. In a carrier-wavesi'gnalling system, a line, a filter connected tosaid line designed to pass cur-- rents having a frequency or frequenciesclose to but outside the frequency band of the carrier waves, a metervariably responsive to the strength of current received from the line'through the filter, an amplifier connected to said line independently ofsaid filter by which the carrier waves are amplified, means for causinga light ray to periodically traverse the indicating face of said meter,means controlled by said meter through the medium of said light ray tocause the generation of impulses of current, a

step-by-step switching device controlled by said impulse generatingmeans to take up a position corresponding to the reading of said meter,and gain controlling means controlled by said step-bystep switchingdevice.

5. In a carrier wave signalling system, a line, a filter connected tosaid line designed to pass currents having a frequency or frequenciesclose to but outside the frequency band of the carrier waves, a metervariably responsive to the strength of current received from the linethrough the filter, an amplifier connected to said line independently ofsaid filter by which the carrier waves are amplified, means for causingalight ray to periodicallytraverse the indicating face of said meter,means controlled by said meter through the medium of said light ray tocause the generation of impulses of current, two step-by-step switchingdevices, means for causing said switching devices to be controlledalternately by said impulse generating means so that one is restored tonormal and reset while the other remains operated, gain controllingmeans controlled by each switching device and switching means wherebythe operated switching device is alone efiective in determining the gainof said circuit;

6. In a carrier wave signalling system, a line, a filter connected tosaid line designed to pass currents having a frequency orfrequenciesclose to but outside the frequency band of the carrier waves, a metervariably responsive to the strength of current received from the linethrough the filter, an amplifier connected to said line independently ofsaid filter by which the carrier waves are amplified, means for causinga light ray toperiodically traverse the indicating face of said meter,progressively-operated means controlled by said meter through the mediumof said light rays to take up a position corresponding to the reading ofsaid meter and gain-controlling means controlled by saidprogressively-operated means.

. TESEO BRUNO DANTE TERRONI.

